Coated phosphor filler and a method of forming the coated phosphor filler

ABSTRACT

An improved coated phosphor filler for an optical device includes a plurality of individual phosphor filler particles, and a coating layer having a plastic substance coated on each of the phosphor filler particles.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a coated phosphor filler for an opticaldevice (e.g., a light emitting diode (LED)), a method for forming acoated phosphor filler and a method for forming a LED making use of thatcoated phosphor filler.

[0002] In the prior art, phosphor fillers including phosphor particlesare used in a broad field of applications, ranging fromelectro-luminescence to photo-luminescence devices. This richapplicability is due to the favourable physical properties of phosphorsuch as a high luminescence efficiency and lifetime as well as due tothe presence of suitable emission colours in the optical emissionspectrum.

[0003] A technological application of such phosphor fillers withincreasing importance is the light emitting diode (LED), which comprisesa LED-chip being electrically connected to a electrically conductingcontact base. The LED-chip usually comprises a semiconductingp-n-junction, in which electrons and holes, which have been injected viaa supply voltage, recombine under light emission. In order to direct thelight emission into the operational direction of the LED-Chip, theLED-Chip is usually encapsulated by an optical dome made of transparentresin which, in turn, may include a phosphor filler by which the lightemitting spectrum of the LED-Chip can be converted as necessary.

[0004] In particular, due to the development of blue emitting LED-chipsand the use of such phosphor fillers, LED-devices providing a widecolour range can be obtained, including the so-called “white LED”, whichcan compete with conventional types of light sources in a broad field ofapplications such as traffic lights and signboards.

[0005] Generally speaking, such phosphor fillers may be based ondifferent types of phosphor compounds, namely stable and unstablephosphor compounds. Stable phosphor compounds may e.g. comprise membersof the garnet family, preferably (YGd)₃Al₅O₁₂ including Ce³⁺-impurities.Unstable phosphor compounds may e.g. comprise SrGa₂S₄:Eu²⁺, SrS:Eu²⁺,(Sr,Ca)S:Eu²⁺, ZnS:Ag.

[0006] The advantage of a phosphor filler in the form of stable phosphorcompound particles is that it is not sensitive to moisture which would,in turn, reduce the reliability of the electrical device, like anLED-Chip encapsulated in an epoxy dome comprising such a phosphorfiller.

[0007] However, it is also known in the prior art that the performanceof devices with unstable phosphor compound can be improved, too, bycoating the phosphor compound material, i.e. the outer surface of theindividual unstable phosphor compound particles, with a protectivecoating film. In particular, said unstable phosphor compound particlesmay be coated with an inorganic coating film including a moisture-proofbarrier material such as aluminium oxide (Al₂O₃), zinc sulphide (ZnS),silicon nitride (Si₄N₃) or the like. In case of such fillers based onunstable phosphor compounds, the inorganic coating film on theindividual-phosphor compound particles provides for a chemical andphotochemical degradation protection of the phosphor compound.

[0008] In the light of the foregoing explanations, the term “phosphorfiller” refers, in the following description, to a plurality of phosphorfiller particles which are either stable phosphor compound particles orunstable phosphor compounds particles coated with an inorganicmoisture-proof coating film.

[0009] From U.S. Pat. No. 4,585,673, a method for forming a protectivecoating film on unstable phosphor compound particles is known, whereinthe protective coating film is formed by a gas-phase chemical vapourdeposition (MOCVD=“metal organic chemical vapour deposition”) on thephosphor compound particles which are suspended in a fluidised bed whichis maintained in a temperature gradient, said protective coating being arefractory oxide such as aluminium oxide.

[0010] U.S. Pat. No. 6,001,477 discloses a method for providing on thesurface of individual unstable phosphor compound particles a continuous,non-particulate coating of a metal or metalloid compound such as siliconor boron by means of a reaction between the metal or metalloid and apolymer capable of chelating ions of the metal or metalloid. Theresulting coating (e.g. a BA-PVM/MA coating) is chemically adhered tothe phosphor compound particles which exhibits improved lumenmaintenance when applied to the inner surface of a lamp envelope.

[0011] U.S. Pat. No. 5,985,175 discloses a method for providing onindividual unstable phosphor compound particles a continuous,non-particulate coating of boron oxide to enhance the quantum efficiencyof the phosphor compound particles under ultraviolet (UV) and vacuumultraviolet (VUV) excitation. The method involves reacting aboron-containing precursor with an oxidizing gas in a fluidised bed ofphosphor particles.

[0012] Furthermore and more generally, EP 0 539 211 B1 discloses amethod for production of a microcapsule type conductive filler, whereinthis conductive filler is dispersed in an epoxy type one-componentadhesive agent.

[0013] A possible structure of a phosphor filler 100 according to theprior art is schematically illustrated in FIG. 2a. The phosphor filler100 comprises a plurality of unstable phosphor compound particles 101,each of the phosphor compound particles 101 being coated with aninorganic coating film 102. The inorganic coating film 102 consists of asuitable moisture-proof barrier material such as e.g. aluminium oxide(Al₂O₃) and has a thickness in the range of about 3 to 4 μm.

[0014] If the thickness of the coating film 102 is large, the coatingfilm 102 provides a significant deterioration of the opticaltransmissibility. On the other hand, if the thickness of the coatingfilm 102 is low, the spacing between neighbouring phosphor compoundparticles 101 is relatively small. Consequently, the probability oflight symbolized by light beams 103, which is e.g. emitted by a LED asdescribed below with reference to FIG. 2b, to be re-absorbed bysurrounding phosphor compound particles 101 is high and, therefore, thebrightness obtained in a LED using this kind of phosphor filler is low.

[0015] A typical LED 200, as schematically illustrated in FIG. 2b,comprises a LED-chip 201, which is mounted on a first electricallyconducting frame 202. Said first electrically conducting frame 202 isprovided with a reflector cup 202 a including a recess in which theLED-chip 201 is mounted. At least two electrodes (not shown), which maybe surface mounted electrodes, are attached on said LED-chip 201, onebeing electrically connected by means of a first wiring 203 to the firstelectrically conducting frame 202, and the other being electricallyconnected by means of a second wiring 204 to a second electricallyconducting frame 205.

[0016] The LED-chip 201 is covered by a drop 206 containing a mixtureconsisting of epoxy and a phosphor filler dispersed therein, said drop206 filling almost the whole recess of the reflector cup 202 a. Thephosphor compound particles of the phosphor filler may be coated with acoating film including a moisture-proof barrier material such asaluminium oxide (Al₂O₃), i.e. they may form a structure as describedabove with respect to FIG. 2a.

[0017] Furthermore, the major upper part of the first and secondelectrically conducting frames 202 and 205 as well as the wholearrangement formed by the LED-chip 201 covered by the drop 206 and thewirings 203 and 204 are encapsulated by an optical dome (or opticallens) 207 formed of transparent epoxy.

[0018] The LED 200 can e.g. be operated as a white light emitting diode,wherein phosphor compound particles in the drop 206 re-emit a broad bandof yellow, yellow-green or red-green light with unabsorbed blue lightfrom the LED-chip 201.

[0019] Two common methods for forming a LED device are schematicallyillustrated in FIG. 3. These methods are generally referred to as the“pre-mix method” (FIG. 3a) and the “pre-dep method” (FIG. 3b).

[0020] In the so-called “pre-dep method”, as can be seen in FIG. 3b, aLED-chip 301 of a LED device 300 is placed inside a reflector cup 302 ofa metal base 303 in a first step. Then the LED-chip 301 is electricallyconnected, by means of wirings 304, to the metal base 303. In the nextstep, a drop 305 containing a mixture of phosphor compound particles 306and epoxy 307 is filled into the reflector cup 302 to cover the LED-chip301. Finally, the whole structure of the drop 305 covering the LED-chip301, the wirings 304 and the metal base 303 is over-moulded with epoxyto form a transparent optical dome 308.

[0021] In contrast to this method, the so-called “pre-mix method”prevents a procedure of covering of the LED-chip 301 in two steps. Toachieve this simplification of the manufacturing process, the LED-chip301 is over-moulded, as can be seen in FIG. 3a, in only one step by anoptical dome 309 containing a pre-mixed mixture of phosphor compoundparticles 310 and epoxy 311.

[0022] Accordingly, whereas the pre-mix method of FIG. 3a simplifies themanufacturing process, the pre-dip method of FIG. 3b provides, due tothe completely transparent optical dome 308, a more efficient lightextraction from the LED-chip 301.

[0023] However, optical devices such as a light emitting diode (LED)including phosphor fillers according to the prior art, i.e. stable orunstable phosphor compound particles being coated with none or only oneprotective coating film, respectively, wherein the coating film consistsof e.g. aluminium oxide, exhibit several shortcomings for the followingreasons:

[0024] (1) A significant basic problem of prior art LED devices of thetype described above is that the phosphor filler, i.e. the individualphosphor compound particles tend to agglomerate. This problem isobserved and equally valid for all type of phosphor fillers discussedabove, that is for stable phosphor compound particles as well as forunstable phosphor compound particles which are coated with an inorganicmoisture-proof coating film. Such an agglomeration leads, however, to anumber of drawbacks in the operating characteristics of the LED device,such as uneven spectral and brightness distribution of the emitted lightover the emitting surface of the device, loss of brightness of the LEDdevice based on re-absorption effects between neighbouring agglomeratedphosphor particles, etc.

[0025] (2) The prior art LED devices containing unstable phosphorcompound particles coated with an inorganic film as phosphor fillerexhibits a relatively poor light extraction efficiency. In other wordsthe amount of light emitted by such a device compared to the amount oflight which would be emitted by a device which does not comprise such aphosphor filler is significantly reduced. This is based on the fact thatthe refractive index of the inorganic coating film, such as aluminiumoxide, differs from the refractive index of the epoxy resin, resultingin that the light entering and passing through the encapsulating domeexperiences several times total reflection at the inorganic coatingfilm—epoxy—interfaces and, thereby, captured within the dome.

[0026] (3) In the prior art LED device 200, the unstable phosphorcompound particles in drop 206 exhibit a relatively large sensitivity tomoisture, which may enter in the drop 206 and attack the unstablephosphor compound particles situated therein. Consequently, the LED 200is subjected to aging effects, so that the reliability of such a priorart LED is relatively low. This effect is especially disadvantageous inapplications such as traffic lights or signboards, which generallyrequire lifetimes of the used optical device of more than 10⁵ h.

[0027] (4) If unstable phosphor compound particles are used in the drop206 of the LED 200 which are individually coated with a protectivecoating film consisting of e.g. aluminium oxide, the protective coatingreduces the optical transmission of the drop 206 and thereby thebrightness of the LED 200. Accordingly, the thickness of this protectivecoating and, consequently, the protection of the unstable phosphorcompound particles are limited.

[0028] Accordingly, the performance of LED's using prior art phosphorfiller is insufficient particularly with respect to the agglomerationproblem, but, at least in case of unstable phosphor compound particlesalso with respect to light extraction, i.e. the brightness of the LED.Accordingly, the reliability of the known LED devices is low.

SUMMARY OF THE INVENTION

[0029] one feature of the present invention is to provide a phosphorfiller and a method for forming a phosphor filler which enable theproduction of optical devices, such as light emitting diodes (LEDs)including laser diodes, with an improved luminous performance andreliability.

[0030] Another feature of the present invention is to provide a lightemitting diode (LED) (or a laser diode) and a method for forming a lightemitting diode (or a laser diode) with improved luminous performance andreliability.

[0031] A further feature of the invention is to provide a phosphorfiller which, whether it is based on stable or unstable phosphorcompounds, can be evenly dispersed in the transparent plastic substance,like epoxy resin of the optical dome of optical devices.

[0032] A further feature of the invention is to provide a phosphorfiller which can be evenly dispersed and which, at the same time,eliminates or at least reduces the disadvantageous effects of a thickinorganic coating film on unstable phosphor compound particles.

[0033] According to one aspect of the invention, there is provided acoated phosphor filler that includes a plurality of individual phosphorfiller particles that are coated with a coating layer having a plasticsubstance, preferably an optically transparent epoxy composition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIGS. 1a and 1 b are schematic illustrations showing preferredembodiments of the coated phosphor filler according to the presentinvention and its effect on the ability of light emission.

[0035]FIGS. 2a-2 b are a schematic illustration of a phosphor filleraccording to the prior art (FIG. 2a) and a schematic illustration of alight emission diode (LED) according to the prior art (FIG. 2b); and

[0036]FIGS. 3a and 3 b show alternative embodiments of a method forforming a light emission diode (LED).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] As will be described in more detail, a coated phosphor fillerincludes a plurality of individual phosphor filler particles that arecoated with a coating layer having a plastic substance, preferably anoptically transparent epoxy composition.

[0038] Due to this structure of the coated phosphor filler, theperformance of an optical device, such as a LED, using such a coatedphosphor filler is significantly improved with respect to lightextraction, i.e. brightness of the LED device, and reliability of theLED device. Another advantage is that an enhanced reliability of theobtained LED is achieved due to a better passivation of the individualphosphor filler particles against elevated temperatures and humidity.

[0039] Since with regard to the optical transmissibility of the phosphorfiller, the thickness of the coating layer made in accordance with oneembodiment of the present invention is not critical, the distancebetween neighbouring phosphor filler particles may be significantlyenhanced without a deterioration of the optical transmissibility orlight extraction. Consequently, the coating film according to thepresent invention may prevent an agglomeration between neighbouringphosphor particles which is a fundamental advantage of the coatedphosphor filler according to the invention. Further, re-absorptioneffects between different phosphor filler particles are prevented andthe luminous performance of the LED device is improved by the use of theinventive coated phosphor filler.

[0040] Furthermore, a significant improvement of the light extractioncan be achieved with an optical device using the coated phosphor fillermade according to the invention. On the one hand, this effect isachieved due to a modification of the refractive indices present betweenthe individual phosphor filler particles and the outer epoxyencapsulation. The inventive structure of the coated phosphor filler isparticularly advantageous because the coating layer includes the plasticsubstance, in particular if including an optically transparent epoxycomposition, provides additional “interfaces” between the individualphosphor filler particles and the outer epoxy encapsulation. This“interfaces”, preferably epoxy-epoxy-interfaces, have the effect toenhance the extraction efficiency of the optical device due to amodification of the refractive index, since such a modification of therefractive index results in a reduction of Fresnel reflection losseswhich might otherwise be significant, since the refractive index of e.g.the used LED-chip is relatively high compared to the refractive index ofthe surrounding epoxy composition. Accordingly, with the presence ofadditional epoxy-epoxy-interfaces between the LED-chip and thesurrounding epoxy encapsulation, these Fresnel reflection losses will bereduced and the light extraction will be enhanced.

[0041] On the other hand, a significant improvement of the lightextraction in an optical device using the inventive coated phosphorfiller is also due to the fact that the need of thick protective layersconsisting of e.g. an inorganic passivation material is eliminated. Suchprotective layers being coated on the individual phosphor particlesresult in a significant deterioration of light extraction, since theyabsorb a significant part of the light being emitted by e.g. the LEDchip. In the present invention, such a protective passivation layer iseither totally dispensable (in case that the phosphor filler particlesare stable phosphor compound particles), or the thickness of such apassivation layer may be significantly reduced (in case the phosphorfiller particles are unstable phosphor compound particles and anadditional, but relatively thin barrier film is provided between thecoating layer and the individual phosphor filler particles).

[0042] According to a preferred embodiment, the inventive coatedphosphor filler structure is applied to phosphor filler particles whichare formed by unstable phosphor compound particles coated with amoisture proof barrier film. In this case the coating layer comprisingthe optically transparent epoxy composition is coated on the barrierfilm. The phosphor compound particles may e.g. comprise at least one ofthe components SrGa₂S₄:Eu²⁺, SrS:Eu²⁺, (Sr,Ca)S:Eu²⁺ and ZnS:Ag.

[0043] In the structure of the coated phosphor filler according to thispreferred embodiment, the basic aim of the barrier film is to protectthe unstable individual phosphor compound particles from aging effectscaused by the surrounding environment such as moisture, therebypreventing any change in the chemical composition of the unstablephosphor compound particles and preserving their quantum efficiency.

[0044] However, according to another preferred embodiment, the inventivecoated phosphor filler structure can also be applied to phosphor fillerparticles which are formed by stable phosphor compound particles whichdo not need to be coated with a moisture proof barrier film. In thiscase, the phosphor filler particles may comprise at least one member ofthe garnet family, preferably (YGd)₃Al₅O₁₂ including Ce³⁺-impurities.

[0045] The basic aim of the coating layer is to prevent an agglomerationof the phosphor filler particles (formed by either one of the aboveembodiments) and to improve the light extraction to enhance thebrightness of the LED device, and also to protect the barrier filmagainst any chemical decomposition effects. Furthermore, the coatinglayer also provides for additional interfaces between the individualphosphor filler particles and the outer epoxy encapsulation as alreadyexplained above, thereby also contributing to an improvement of thelight extraction and the brightness of the optical device.

[0046] Said barrier film is preferably formed of an inorganicpassivation material, which may include a material selected from thegroup consisting of aluminium oxide, silicon monoxide, zinc sulphide orsilicon nitride.

[0047] The thickness of the coating layer is preferably in the range of2 to 6 μm, more preferably 3 to 5 μm.

[0048] The thickness of the moisture-proof barrier film is preferably inthe range of 0.1 to 2 μm. In particular, the thickness of said coatinglayer is at least twice the thickness of said barrier film. This isparticular advantageous, since due to the optical transparency of thecoating layer, the thickness of said coating layer is less critical thanthe thickness of the barrier film, the latter absorbing a relativelylarge amount of light emitted by e.g. the LED chip. On the other hand,the large thickness of the coating layer provides for an effectiveprotection of the individual phosphor filler particles againstagglomeration.

[0049] The thickness of said coating layer may also be 2 to 10 times thethickness of said barrier film.

[0050] Said phosphor filler particles preferably have a sphere-likeshape, resulting in a relatively easy preparation procedure.Furthermore, such a shape represents an optimum optical and geometricalstructure and makes high packing densities in very thin coating layerswith relatively low light-scattering possible.

[0051] Said epoxy composition is preferably containing hydrophobicresidues forming a moisture-repellent barrier, providing for anadditional moisture protection of the individual phosphor fillerparticles.

[0052] According to another aspect of the invention, in a method forforming a coated phosphor filler, in particular for use in lightemitting diodes, comprising a plurality of individual phosphor fillerparticles, said phosphor filler particles are coated with a coatinglayer comprising a plastic substance.

[0053] According to a preferred embodiment, unstable phosphor compoundparticles are used as said phosphor filler particles and said step ofcoating said phosphor filler particles with a coating layer furthercomprises the steps of coating said unstable phosphor compound particleswith a moisture proof barrier film, and coating the outer surface ofsaid moisture proof barrier film with said coating layer.

[0054] According to a preferred embodiment, said step of coating saidunstable phosphor compound particles with a moisture proof barrier filmis performed by chemically forming said moisture proof barrier film insolution. Said step of coating the outer surface of said moisture proofbarrier film with said coating layer may be performed by physicallydepositing said coating layer on said moisture proof barrier film.

[0055] According to another aspect of the invention, a method forforming a light emitting diode (LED) comprises the steps of mounting aLED-chip on a contact base, electrically connecting said LED-chip to afirst and a second electrically conducting frame, covering said LED-chipwith a coated phosphor filler, said coated phosphor filler including aplurality of individual phosphor filler particles, wherein said coatedphosphor filler is subjected to a pre-treatment, wherein said phosphorfiller particles are coated with a coating layer comprising a plasticsubstance.

[0056] According to another preferred embodiment, unstable phosphorcompound particles are used as said phosphor filler particles and saidstep of coating said phosphor filler particles with a coating layerfurther comprises the steps of coating said unstable phosphor compoundparticles with a moisture proof barrier film, and coating the outersurface of said moisture proof barrier film with said coating layer.

[0057] According to a further preferred embodiment, the step of coveringsaid LED-chip in said first electrically conducting frame with a coatedphosphor filler further comprises the steps of dispensing said LED-chipwith a drop of said coated phosphor filler in a reflector cup providedin said first electrically conducting frame, and over-moulding said dropand at least a part of said first electrically conducting frame with anoptical dome consisting of an optically transparent epoxy.

[0058] According to another preferred embodiment, the step of coveringsaid LED-chip in said first electrically conducting frame with a coatedphosphor filler further comprises the steps of forming a mixture betweena plurality of said individual phosphor filler particles and anoptically transparent epoxy, and over-moulding said LED-chip and atleast a part of said first electrically conducting frame with saidmixture to form an optical dome.

[0059] According to a further aspect of the invention, a mixture isprovided which is prepared in particular for use in the above method.Said mixture is a mixture of a plurality of said individual phosphorfiller particles and an optically transparent epoxy, wherein saidphosphor filler particles are coated with a coating layer comprising aplastic substance and is advantageous in so far, as the necessarycompound does not have to be individually prepared in each manufacturingprocess to obtain the above desired effects according to the presentinvention.

[0060] According to another aspect of the invention, a light emittingdiode (LED) comprises a LED-chip mounted on a contact base, saidLED-chip being electrically connected to a first and a secondelectrically conducting frame, and a coated phosphor filler, said coatedphosphor filler including a plurality of phosphor filler particles andcovering said LED-chip, wherein said phosphor filler particles arecoated with a coating layer comprising a plastic substance.

[0061] According to a preferred embodiment, said LED-chip is coveredwith a drop of said coated phosphor filler in a reflector cup providedin said first electrically conducting frame, and said drop and at leasta part of said first electrically conducting frame are over-moulded withan optical dome consisting of an optically transparent epoxy.

[0062] According to another preferred embodiment, said LED-chip and atleast a part of said first electrically conducting frame areover-moulded with a mixture between a plurality of said individualphosphor filler particles and an optically transparent epoxy, saidmixture forming an optical dome.

[0063] According to another preferred embodiment, an optical dome isprovided covering said LED-chip and consisting of an epoxy material,providing for a protection of the whole arrangement without disturbingits optical properties.

[0064]FIG. 1a and FIG. 1b demonstrate in a schematic view the effect ofthe inventive structure of the coated phosphor composition on theability of light emission.

[0065]FIG. 1a shows a preferred embodiment of a coated phosphor filler10 according to the present invention. The coated phosphor filler 10includes a plurality of stable phosphor compound particles 11, which mayinclude at least one member of the garnet family, preferably(YGd)₃Al₅O₁₂ including Ce³⁺-impurities.

[0066] The individual phosphor compound particles 11 have a sphere-likeshape, and each phosphor compound particle has a diameter in the rangeof 10±5 μm. Each of the phosphor compound particles 11 is coated with acoating layer 12 comprising a plastic substance, preferably an opticaltransparent epoxy. According to this preferred embodiment of theinvention, the coating film is 3 to 4 μm thick.

[0067] Preferably, the coating layer 12 is coated on the individualphosphor particles by physically depositing the coating film on theindividual phosphor compound particles. This may e.g. be performed by animmersion of the phosphor compound particles in the epoxy, followed by adrying period.

[0068] The epoxy composition forming the coating layer 12 preferablycontains hydrophobic residues, which constitute an additionalmoisture-repellent barrier and, thereby, provide for an additionalprotection of the stable phosphor compound particles 11.

[0069] Experiments have shown that phosphor fillers coated with acoating film according to the invention do not tend to agglomerate whenthe phosphor filler particles are dispersed in the epoxy used e.g. toform a transparent optical dome above a LED-chip.

[0070] Furthermore, since the coating layer 12 is formed of atransparent epoxy material, it does not deteriorate the light extractionof the optical device, e.g. the LED-chip. As can be seen in FIG. 1a,light beams symbolized by arrows 13 are extracted from a particularphosphor particle without being disturbed by the surrounding phosphorparticles 11 or by the coating film 12. Consequently, the brightness ofan optical device, such as a LED, using the phosphor filler according tothe invention 10 is enhanced.

[0071]FIG. 1b shows another preferred embodiment of a coated phosphorfiller 20 according to the invention. In this embodiment, the coatedphosphor filler includes a plurality of unstable phosphor compoundparticles 21, which may e.g. comprise at least one of the componentsSrGa₂S₄:Eu²⁺, SrS:Eu²⁺, (Sr,Ca)S:Eu²⁺ and ZnS:Ag.

[0072] Each individual phosphor compound particle is coated with a film22 consisting of a suitable moisture-proof barrier material, preferablyan inorganic passivation material such as e.g. aluminium oxide (Al₂O₃).Said barrier material may also contain other types of passivationmaterials such as silicon monoxide (SiO), zinc sulphide or siliconnitride (Si₃N₄).

[0073] The coating of the unstable phosphor compound particles with themoisture proof barrier film 22 is preferably performed by the so-calledWet Chemical process. Again, the coating layer 23 may be coated on themoisture proof barrier film 22 by physically depositing the coating filmon the moisture proof barrier film 22, which may e.g. be performed by animmersion of the phosphor compound particles coated with the film 22 inepoxy and following by a drying period.

[0074] According to the invention, on each moisture-proof barrier film22 a coating layer 23 is coated, which corresponds to the coating layer12 in the coated phosphor filler 10 of FIG. 1a, and which comprises aplastic substance, preferably an optical transparent epoxy. In apreferred embodiment, oxygen-containing functional groups of the epoxycompounds chemically interact, i.e. form a chemical bond, with the metalions (e.g. aluminium ions) of the barrier material (e.g. aluminiumoxide, Al₂O₃). Moreover, the coating layer 23 is again preferably formedby hydrophobic epoxy compounds which constitute an additionalmoisture-repellent barrier providing for an additional protection of theunstable phosphor compound particles 21. Consequently, according to thispreferred embodiment of the invention, the thickness of the barrier film22 can be reduced compared to the thickness of the barrier film 102 ofthe prior art phosphor filler 100 as shown in FIG. 2a withoutdeteriorating the moisture protection of the unstable phosphor compoundparticles 21. Further, an optical device such as a LED having an opticaldome with phosphor filler particles dispersed therein, wherein thephosphor filler particles comprise a thin barrier film according to thispreferred embodiment of the invention, provides for superior lightemitting characteristics due to the thinner moisture-proof barrier film.

[0075] Preferably, the thickness of said coating layer 23 is relativelylarge compared to the thickness of said barrier film 22. Morespecifically, the thickness of the coating layer 23 can be at leasttwice, or preferably 2 to 10 times the thickness of the barrier film 22.

[0076] In particular, the thickness of the moisture-proof barrier film22 can be in the range of about 0.1 to 2 μm, whereas the coating layer23 may have a thickness of about 2 to 6 μm and, further preferably, of 3to 5 μm.

[0077] Similar to the embodiment shown in FIG. 1a, the phosphor fillercomprising unstable phosphor compound particles 21 coated with themoisture-proof barrier film 22, which, in turn, are coated with thecoating layer according to this embodiment of the invention shown inFIG. 1b, does not tend to agglomerate when the phosphor filler particlesare dispersed in the epoxy used e.g. to form a transparent optical domeabove a LED-chip.

[0078] As can be seen from FIG. 1a and FIG. 1b, the distance betweenneighbouring phosphor filler particles 11 or 21 and 22 in the coatedphosphor filler 10 and 20, respectively, is significantly enhancedcompared to the prior art phosphor filler illustrated in FIG. 2a.Consequently, the ratio of light (symbolized in FIG. 1a and FIG. 1b bylight beams 13 or 24, respectively) emitted through the gap which isleft between surrounding phosphor filler particles 11 or 21 and 22 isenhanced, leading to an improvement of the brightness achieved in a LEDusing this kind of coated phosphor fillers.

[0079] Since the coated phosphor filler 10 of FIG. 1a comprises stablephosphor compound particles 11 and, therefore, does not need amoisture-proof barrier film around the particles 11, the phosphor filler10 exhibits a higher optical transmissibility compared to the phosphorfiller 20 of FIG. 1b which comprises unstable phosphor compoundparticles 21 coated with the moisture-proof barrier film 22. However, inthe phosphor filler 20 of FIG. 1b, the thickness of the additionalbarrier film 22 including the moisture-proof barrier material may besignificantly lower than in the prior art, while still giving anadditional protection to the unstable phosphor compound particles 21against humidity and corresponding aging effects by means of the coatinglayer 23.

[0080] The coated phosphor filler according to the present invention maybe used to form optical devices, such as a white LED as illustrated inFIG. 2b, by means of conventional methods, such as the “pre-mix” methodor the “pre-dep” method described above with respect to FIGS. 3a,b. Dueto the inventive structure of the coated phosphor filler, theperformance of such an optical device is significantly improved withrespect to light extraction and brightness of the optical device, butalso with respect to the reliability of the optical device.

What is claimed is:
 1. A coated phosphor filler, comprising a pluralityof individual phosphor filler particles; a coating layer coated on thephosphor filler particles, wherein the coating layer comprises a plasticsubstance.
 2. The coated phosphor filler according to claim 1, whereinthe plastic substance comprises an optically transparent epoxycomposition.
 3. The coated phosphor filler according to claim 1, whereinthe phosphor filler particles are stable phosphor compound particles. 4.The coated phosphor filler according to claim 3, wherein the phosphorfiller particles comprise at least one member of the garnet family,preferably (YGd)₃Al₅O₁₂ including Ce³⁺-impurities.
 5. The coatedphosphor filler according to claim 1, wherein the phosphor fillerparticles are unstable phosphor compound particles coated with amoisture-proof barrier film, the coating layer being provided on theouter surface of said barrier film.
 6. The coated phosphor filleraccording to claim 5, wherein the phosphor compound particles compriseat least one of the components SrGa₂S₄:Eu²⁺, SrS:Eu²⁺, (Sr,Ca)S:Eu²⁺ andZnS:Ag.
 7. The coated phosphor filler according to claim 5, wherein saidbarrier film is formed of an inorganic passivation material.
 8. Thecoated phosphor filler according to claim 7, wherein said inorganicpassivation material includes a material selected from the groupconsisting of aluminium oxide, silicon monoxide, zinc sulphide orsilicon nitride.
 9. The coated phosphor filler according to claim 1,wherein the thickness of the coating layer is in the range of 2 to 6 μm,preferably 3 to 5 μm.
 10. The coated phosphor filler according to claim5, wherein the thickness of the moisture-proof barrier film is in therange of 0.1 to 2 μm.
 11. The coated phosphor filler according to claim5, wherein the thickness of said coating layer is at least twice thethickness of said barrier film.
 12. The coated phosphor filler accordingto claim 5, wherein the thickness of said coating layer is 2 to 10 timesthe thickness of said barrier film.
 13. The coated phosphor filleraccording to claim 2, said epoxy composition includes hydrophobicresidues forming a moisture-repellent barrier.
 14. A method for forminga coated phosphor filler, comprising coating each of a plurality ofindividual phosphor filler particles with a coating layer comprising aplastic substance.
 15. The method according to claim 14, wherein thephosphor filler particles are unstable phosphor compound particles,wherein said step of coating further comprises the steps of: coatingsaid unstable phosphor compound particles with a moisture proof barrierfilm; and coating the outer surface of said moisture proof barrier filmwith said coating layer.
 16. The method according to claim 15, whereinsaid step of coating said unstable phosphor compound particles with amoisture proof barrier film is performed by using the Wet Chemicalprocess.
 17. The method according to claim 16, wherein said step ofcoating the outer surface of said moisture proof barrier film with saidcoating layer is performed by physically depositing said coating layeron said moisture proof barrier film.
 18. The method according to claim15, wherein an inorganic passivation material is used as said barriermaterial.
 19. A light emitting diode (LED) comprising a LED-chip mountedon a contact base, said LED-chip being electrically connected to a firstand a second electrically conducting frame; and a coated phosphorfiller, said coated phosphor filler including a plurality of phosphorfiller particles and covering said LED-chip, wherein said phosphorfiller particles are coated with a coating layer comprising a plasticsubstance.
 20. The light emitting diode (LED) according to claim 19,wherein said LED-chip is covered with a drop of said coated phosphorfiller in a reflector cup provided in said first electrically conductingframe, and wherein said drop and at least a part of said firstelectrically conducting frame are over-moulded with an optical domeconsisting of an optically transparent epoxy.
 21. The light emittingdiode (LED) according to claim 19, wherein said LED-chip and at least apart of said first electrically conducting frame are over-moulded with amixture of a plurality of said individual phosphor filler particles andan optically transparent epoxy, said mixture forming an optical dome.22. The light emitting diode (LED) according to claim 19, wherein theplastic substance is an optically transparent epoxy composition.
 23. Thelight emitting diode (LED) according to claim 19, wherein said phosphorfiller particles are unstable phosphor compound particles coated with amoisture-proof barrier film, the coating layer being provided on theouter surface of said barrier film.
 24. The light emitting diode (LED)according to claim 19, further comprising an optical dome, said opticaldome covering said LED-chip and consisting of an epoxy material.
 25. Thelight emitting diode (LED) according to claim 19, said barrier materialincludes an inorganic passivation material.
 26. The light emitting diode(LED) according to claim 25, said inorganic passivation materialincluding a material selected from the group consisting of aluminiumoxide (Al₂O₃), silicon monoxide (SiO), zinc sulphide (ZnS) or siliconnitride (Si₃N₄).
 27. The light emitting diode (LED) according to claim19, said epoxy composition includes hydrophobic residues forming amoisture-repellent barrier.